Improve the package registration documentation by adding the descriptions for GitHub beginners

Please update the documentation to be more friendly to GitHub beginners.
https://optuna.github.io/optunahub-registry/recipes/002_registration.html

For example, introducing instructions like the ones below is an idea of improvements.
```
1. git clone
2. cp -r template/ package/samplers/hoge_sampler
3. ...

ref: https://devguide.python.org/
```